Research Article Sodium Tripolyphosphate and Polyphosphate...

Hindawi Publishing CorporationJournal of EngineeringVolume 2013, Article ID 930832, 4 pageshttp://dx.doi.org/10.1155/2013/930832

Research ArticleSodium Tripolyphosphate and Polyphosphate as DispersingAgents for Alumina Suspensions: Rheological Characterization

L. Piani and A. Papo

Dipartimento di Chimica, Fisica e Ambiente, Universita degli Studi di Udine, Via del Cotonificio 108, 33100 Udine, Italy

Correspondence should be addressed to L. Piani; [email protected]

Received 24 January 2013; Accepted 29 May 2013

Academic Editor: Paul Luckham

Copyright © 2013 L. Piani and A. Papo.This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In the present work, the influence of addition of some dispersing agents employed for maximizing the solid loading of aluminaconcentrated suspensions has been investigated. Two commercially available deflocculants were used: a sodium tripolyphosphateand a sodium polyphosphate. Rheological tests were carried out at 25∘C under continuous flow conditions by using the rate-controlled coaxial cylinder viscometer Rotovisko-Haake 20, system M5-osc., measuring device SV2P with serrated surfaces. Theapplication of rheological techniques permitted the evaluation of the efficiency and the optimum dosage of the dispersing agentsemployed upon the alumina slips examined, that is, the critical concentration at which the maximum reduction in viscosity isdetected.

1. Introduction

Alumina is widely used in a lot of industrial applicationswhich presuppose the employment of concentrated suspen-sions. For the preparation of ceramic suspensions bymeans oftechniques such as slip casting and tape casting, submicron-sized ceramic powders are needed. Since their flow character-istics are usually of crucial importance, they have to be con-trolled in order to yield a final product with the best proper-ties as well as to improve the economics of the process and tooptimise energy requirements. In general, the rheologicalbehavior of concentrated suspensions is strongly affected notonly by solid loading, particle morphology, and particle sizedistribution but also by particles interactions, which in theirturn are dependent of chemical factors, that is, of pH and/oraddition of dispersing agents.

Since very fine ceramic particles spontaneously agglom-erate owing to attractive van der Waals forces, they must bedispersed in liquid phase using suitable dispersing agents.The addition of dispersants to suspension can modify theirrheological behavior by reducing or enhancing the attractiveforces among particles, thus producing dispersion or aggre-gation, respectively. Generally, suspensions can be dispersedby electrostatic stabilization mechanism; moreover, theemployment of oligomeric or polymeric dispersants induces

a mechanism of steric stabilization, which is superimposedto the electrostatic one; here, reference is made of Napper [1]and Cesarano and Aksay [2, 3]. The addition of dispersantsto alumina suspensions induces particle repulsion and hencea viscosity decrease until the saturation adsorption limit ofdispersing agent is reached; this phenomenon takes place incorrespondence to a critical dispersant concentration (opti-mum dosage) at which electrostatic or electrosteric stabiliza-tion occurs. At the optimum dosage, all of the dispersants isvirtually adsorbed, and there are negligible amount of freedeflocculant remaining in the continuous phase: the slurryshows the minimum viscosity value. Past the adsorption sat-uration limit, a further addition of dispersant leads to excessdeflocculant in the solution, which is not adsorbed on thesuspended particles, thus bringing about destabilization, floc-culation, and, therefore, an increase of suspension viscosity.

The present work is aimed at studying the influence of theaddition of a sodium tripolyphosphate and a sodium poly-phosphate on the flow characteristics of suspensions preparedwith an alumina powder of high purity.

There is a significant amount of scientific literature onalumina suspensions rheology. In this paper, a mention ismade of some of themost recent articles: DeVasconcelos et al.[4], Lu andKessler [5], Papo andPiani [6], Pradhan andBhar-gava [7], and Tomasik et al. [8].

2 Journal of Engineering

Table 1: Description of the alumina powder employed.

Trade name: High Purity Alumina AKP-15 by Sumitomo ChemicalCo. Ltd. (Japan)Crystal form: 𝛼-Al2O3

Purity: > 99.99%Mean particle size: 0.68𝜇mParticle size distribution <1 𝜇m: 83%Loose bulk density: 0.9 g/cm3

Tapped bulk density: 1.3 g/cm3

BET specific surface area: 3.8m2/gImpurity analysis: Si 22 ppm; Na 5 ppm; Mg 6 ppm; Cu < 1 ppm;Fe 19 ppm

Sodium tripolyphosphate and sodium polyphosphate areanionic inorganic dispersants. Inorganic dispersants haveattracted great attention as ceramic processing aids becausethe surfactants themselves can be incorporated into theceramicmatrix (Exarhos et al. [9]).When organic surfactantsare used in the colloidal ceramic processing (mixing, dispers-ing, and forming) stages, they have to be removed later bythermal decomposition. This process causes materials toshrink and generates gaseous wastes. The inorganic disper-sants as an alternative processing aid will eliminate or reducethese problems because the majority of the inorganic surfac-tants will be converted to ceramic materials.

2. Experimental Methodology

2.1. Materials. The suspensions were prepared with a vanestirrer (Ultra-Turrax T50, Janke & Kunkel, IKA-Labortech-nik) from deionized water and an alumina powder ofhigh purity, whose characteristics supplied by producer arereported in Table 1.

Plain suspensions with increasing solid content were pre-pared starting from 62.5 wt% of alumina. At last 77.5 wt% wasfound to be the highest solid content of alumina in the sus-pension that can be loaded into the viscometric cup withoutdeflocculant (the highest measurable viscosity referred to ourrheometer has been taken as reference).

Two commercial condensed phosphates were testedas potential dispersing agents for alumina suspensions onaccount of their satisfactory dispersing capacity and low cost:a sodium tripolyphosphate (P

2O5/Na2O = 0.6; molecular

weight = 367.86; in the following: TPP) and a sodiumpolyphosphate (commercial name: hexametaphosphate)(P2O5/Na2O = 0.9; average molecular weight = 1733.39; in

the following: PP) by Sigma-Aldrich. Both dispersants wereemployed within a large range of concentration (0.005 to1 wt%) and were added to a 77.5 wt% aqueous alumina sus-pension.

2.2. Apparatus and Procedures. Rheological measurementswere carried out using the rate-controlled coaxial cylinderviscometer Rotovisko-Haake 20, systemM5-Osc., measuringdevice SV2Pwith serrated surfaces.The temperaturewas kept

0 100 200 300 400

𝜂(m

Pa·s)

106

105

104

103

102

10

0.00 wt% 0.10 wt%0.01 wt% 0.30 wt%0.03 wt% 0.50 wt%0.05 wt% 1.00 wt%

�� (s−1)

Figure 1: Apparent viscosity (𝜂) versus shear rate ( 𝛾) flow curvesfor the 77.5 wt% alumina + sodium tripolyphosphate suspensionsstudied.

strictly constant at 25.0 ± 0.1∘C. Flow curves were drawnunder continuous flow conditions by changing the shear ratefrom 0 to 389 s−1 at the 3.24 s−2 constant shear acceleration;each suspension was initially sheared at a high constant shearrate in order to suppress the previous rheological history ofthe sample tested. Each sample was analyzed in duplicate;each determination was repeated twice.

3. Results and Discussion

Concentrated alumina suspensions clearly show non-Newto-nian flow characteristics. Time-dependent effects under con-tinuous shearing conditions were negligible with aluminasuspensions added with deflocculants. A rather good super-position of up and down curves was observed in the wholerange of deflocculant concentration. Hence, in the presentwork, attention has been essentially focused on the shear-dependent behavior of the slips studied.

Figures 1-2 report the apparent viscosity (𝜂) versus shearrate ( 𝛾) flow curves obtained for the alumina slips investi-gated.

From examination of the curves, we can state that theaddition of either dispersant does not modify the rheologicalbehavior of plain suspension within all the deflocculant con-centration range examined but, on the other side, producesa viscosity collapse made evident by adding a slight amountof deflocculant. Viscosity then decreases until reaching opti-mum dosage where the slurry shows the minimum viscosityvalue. Any further addition of dispersant leads to an increaseof suspension viscosity. Between the dispersants employed,PP shows the more effectiveness (see Figures 1–3), becauseit produces a viscosity collapse at lower concentration withrespect to TPP.

Journal of Engineering 3

0 100 200 300 400

𝜂(m

Pa·s)

106

105

104

103

102

10

0.00 wt% 0.05 wt%

0.03 wt%0.01 wt%

0.10 wt%0.005 wt%1.00 wt%

�� (s−1)

Figure 2: Apparent viscosity (𝜂) versus shear rate ( 𝛾) flow curves forthe 77.5 wt% alumina + sodium polyphosphate suspensions studied.

Moreover, PP shows lower optimal dosage (0.05wt%)than TPP (0.10 wt%). However both dispersants do notexhibit a large stabilization range of concentrations beyondoptimumdosage but give rise into the suspension to a gradualincrease of viscosity becoming sharper only with TPP above0.30wt% of concentration.

TPP and PP dispersing mechanisms in alumina suspen-sions are determined by interactions between their ionisedmolecules and surface of alumina particles. The negativelycharged phosphate ions are adsorbed at the alumina surfacesites through hydrogen bonding. This produces a negativecharge on the particles and electrostatic repulsion that causesthe decrease of viscosity in the suspension. However, onlypure electrostatic repulsion cannot explain the decrease ofviscosity, but a steric effect associated with the dispersantchain length must be considered.

Sodium polyphosphate is a polymetaphosphate and haslinear chain conformation. PP, which obviously has longerchain than TPP, is bound to the particle surface through theend groups, andmost of themiddle-chain regions do not par-ticipate in the bonding (Liu et al., [10]).This implies that neg-atively chargedPP chains extend from the particle out into thesolution and provide the additional steric repulsion forces.Therefore, the stabilization of alumina suspensions can bebest described by electrosteric interactions (higher in PPrather than in TPP), thus justifying the better effectiveness ofPP between phosphate dispersants here employed.

4. Conclusions

Alumina concentrated suspensions examined show a shear-thinning behavior. Shear-thinning behavior does not changein the presence of the dispersants (sodiumpolyphosphate andsodium tripolyphosphate) that have been used in this work.

0.01 0.1 1 100.001

TPP PP

cd (wt%)

𝜂(m

Pa·s)

104

103

102

10

Figure 3: Apparent viscosity (𝜂) versus deflocculant concentration(𝑐𝑑

) for 77.5 wt% alumina suspension as function of the dispersantnature ( 𝛾 = 200 s−1).

Between the dispersants adopted, PP seems to be themore effective dispersing agent for the alumina slips investi-gated because it exhibits the higher efficiency (collapse inviscosity) at lower concentration with respect to the otherdeflocculant employed.

Nevertheless, an optimum dosage can be found for bothdispersants adopted. The lower optimum dosage evidencedby sodium polyphosphate can be ascribed to a substantialsteric contribution due to the adsorbed polyphosphate mol-ecules which form in solution oligomeric chains. On theother side, both dispersants do not give rise to any appre-ciable stabilization range of concentrations after reaching thedeflocculant adsorption saturation (optimum dosage).

Symbols

𝑐𝑑: Deflocculant concentration, wt%.

Greek Letters

𝛾: Shear rate, s−1𝜂: Apparent viscosity, mPa s.

References

[1] D. H. Napper, Polymeric Stabilization of Colloidal Dispersions,Academic Press, London, UK, 1983.

[2] J. Cesarano III, I. A.Aksay, andA. Bleier, “Stability of aqueous𝛼-Al2

O3

slurry with poly (methacrylic acid) polyelectrolyte,” Jour-nal of the American Ceramic Society, vol. 71, no. 4, pp. 250–255,1988.

[3] J. Cesarano III and I. A. Aksay, “Processing of highly concen-trated aqueous 𝛼-alumina suspensions stabilized with polyelec-trolytes,” Journal of the American Ceramic Society, vol. 71, no. 12,pp. 1062–1067, 1988.

[4] C. L. de Vasconcelos, T. N. C. Dantas, M. R. Pereira, and J. L.C. Fonseca, “Rheology of concentrated alumina-polyelectrolyte

4 Journal of Engineering

systems,” Colloid and Polymer Science, vol. 282, no. 6, pp. 596–601, 2004.

[5] K. Lu and C. Kessler, “Colloidal dispersion and rheology studyof nanoparticles,” Journal of Materials Science, vol. 41, no. 17, pp.5613–5618, 2006.

[6] A. Papo and L. Piani, “Rheological properties of aluminaslurries: effect of deflocculant addition,” Particulate Science andTechnology, vol. 25, no. 4, pp. 375–380, 2007.

[7] M. Pradhan and P. Bhargava, “Influence of sucrose additionon rheology of alumina slurries dispersed with a polyacrylatedispersant,” Journal of the American Ceramic Society, vol. 88, no.4, pp. 833–838, 2005.

[8] P. Tomasik, C. H. Schilling, R. Jankowiak, and J.-C. Kim, “Therole of organic dispersants in aqueous alumina suspensions,”Journal of the European Ceramic Society, vol. 23, no. 6, pp. 913–919, 2003.

[9] G. J. Exarhos, K. F. Ferris, D. M. Friedrich, and W. D. Samuels,“Atomic and molecular processing of electronic and ceramicmaterials: preparation, properties, and characterization,” inPro-ceedings of the Materials Research Society, pp. 127–134, MaterialsResearch Society, Pittsburg, PA, USA, 1988.

[10] J. Liu, L.-Q. Wang, W. D. Samuels, and G. J. Exarhos, “Aggrega-tion and dispersion of colloidal suspensions by inorganic sur-factants: effect of chemical speciation andmolecular conforma-tion,” Journal of Physical Chemistry B, vol. 101, no. 41, pp. 8264–8269, 1997.

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